The thermal properties of stratum in the earth's interior are the most basic physical parameters, in the study of the thermal structure and thermal evolution in the earth's interior, and the geodynamics. The thermal properties vary under different temperatures and pressures. As such, it is of great significance to study the determination of the thermal properties of rocks under different confining pressures.
At present, the existing methods and systems for determining the thermal properties of rocks under high pressure conditions, usually involve assembling a detector (including a heat source and a temperature sensor) and the rock sample together before setting them in a pressure vessel. The confining pressure in the pressure vessel is then increased to a predetermined value by a booster pump and maintained for a period. When the temperature of the whole system reaches equilibrium, the determination system is turned on so as to perform electrical heating and monitor the change in internal temperature of the rock, and thereby the determination of thermal properties under different confining pressures is achieved.
In the methods and systems mentioned above, active electrical heating (for example, by passing a constant current through a heating wire) is required to serve as a “heat source” for the determination of thermal properties by a transient method. As such, the heat source and the temperature sensor must be both mounted inside the rock. It makes the system complicated. Moreover, the determination of thermal properties has an extreme high requirement on the constancy of temperature, but it is difficult to perform the determination under a constant temperature condition in laboratory since the system is usually exposed to the air. As it is difficult to control the ambient temperature fluctuation, results of the determination are affected greatly most of the time.
In this kind of method and system, active electrical heating (for example, by passing a constant current through a heating wire) is required to serve as the “heat source” for the determination of thermal properties by a transient method.
Our experiments show that, the adiabatic stress derivatives of the temperature (ΔT/Δσ) of common crustal rocks are usually low (of only 2-6 mK/MPa), while those of pressure mediums (for example, seawater) can reach 17.67 mK/MPa, an order of magnitude higher than that of the common crustal rocks. Therefore, after an instant increase of confining pressure, there will be a temperature difference between the rock sample and the seawater as pressure medium. As such, the present invention has achieved the determination of thermal properties (thermal conductivity, thermal diffusivity, and volumetric heat capacity) under high pressure conditions, by real-time monitoring the temperature changes of the rock sample (in the center and on the surface) and the seawater as pressure medium during the instant increase of confining pressure in the pressure vessel, and employing a finite element numerical inversion method.